1. Field of the Invention
The invention relates to an optical disc apparatus for recording an information signal such as a video signal or the like on an optical disc and for reproducing the recorded signal.
2.Description of Background Information
An optical disc apparatus is provided with a tracking servo apparatus to accurately read out the signal recorded on tracks of an optical disc by a pickup. As a system of realizing the tracking servo apparatus, there are generally a three-beam system and a push-pull system.
According to the three-beam system, a main beam spot to be located on track and two sub beam spots are formed in a manner that the sub-beams are arranged obliquely and symmetrically in front and rear, respectively, of the main beam spot as a center of symmetry. The amounts of reflection light of the two sub beam spots are detected by photodetectors, a difference between the reflection light amounts is generated as a tracking error signal, and an objective lens is controlled so as to trace the track in accordance with the tracking error signal.
According to the push-pull system, a reflection light amount by one beam spot to be located on the track is detected by a split photodetector (split into two halves) and a difference between outputs of the two detector halves is generated as a tracking error signal.
In an optical disc apparatus capable of the reproduction only, a tracking servo apparatus of the three-beam system is generally used because the stable operation is obtained. On the other hand, in case of a recordable optical disc apparatus, since three operating modes for recording, reproducing, and erasing are executed, the push-pull system is used as a system for tracking servo because a level fluctuation of the tracking error signals when no signal is recorded and after the signal was recorded is small in each operating mode.
Since the tracking error signal by the three-beam system represents the light and dark states of the reflected light, a signal amplitude is extremely small for the unrecorded portion of the disc as shown in FIG. 1. In the reproduction only optical disc apparatus having the tracking servo apparatus of the three-beam system, there is a problem such that when the pickup reaches the unrecorded portion of the disc, no servo loop is formed because the amplitude of the tracking error signal suddenly decreases and the beam spot by the pickup rush away.
In case of writing or reading the data which was managed on the disc on a sector unit basis, a possibility such that the pickup reaches the unrecorded portion is small. In over writing type recording operation, in case of handling continuous data as an information signal, there is a possibility such that an unrecorded portion occurs in a junction portion with the portion in which the signal has already been recorded.